1. Field of the Invention
The present invention relates to a method for manufacturing a display device. More particularly, the present invention is concerned with a method for manufacturing a display device using an organic electroluminescent element, which method is advantageous in that, even when a light emitting layer is formed by a heat transfer method, the resultant display device can maintain satisfactorily high light emission efficiency and long brightness-half-life.
2. Description of Related Art
An organic electroluminescent element utilizing electroluminescence of an organic material has an organic layer composed of a stack of a hole transport layer, a light emitting layer, and others disposed between a bottom electrode and a top electrode, and has attracted attention as a light emitting element which can achieve light emission with high luminance by low-voltage direct driving.
A full color display device using such an organic electroluminescent element has organic electroluminescent elements individually having R (red), G (green), and B (blue) colors arranged on a substrate. In the manufacture of this display device, at least a light emitting layer composed of organic luminescent materials emitting light of the respective colors is necessarily patterned per electroluminescent element. Patterning the light emitting layer is performed by a shadow masking method in which a luminescent material is deposited or applied through a mask composed of, for example, a sheet having an opening pattern formed, or by an ink-jet method.
The patterning by a shadow masking method has problems in that it is difficult to form a finer opening pattern in the mask, and that patterning with high positioning precision for the electroluminescent element region is difficult due to the deflection or elongation of the mask, thus making it difficult to obtain further scaled-down organic electroluminescent elements with an increased degree of integration. In addition, a functional layer composed mainly of a previously formed organic layer is likely to suffer a damage due to the contact of the mask having an opening pattern formed, thus lowering the manufacturing yield.
Further, the patterning by an ink-jet method has a problem in that its limited patterning precision makes it difficult to obtain scaled-down electroluminescent elements with an increased degree of integration and to achieve a substrate having an increased size.
For solving the problems, as a new patterning method for a light emitting layer composed of an organic material or other functional layers, a transfer method using an energy source (heat source) (i.e., heat transfer method) has been proposed. The manufacture of a display device using the heat transfer method is, for example, conducted as follows. A bottom electrode is first formed on a substrate for display device (hereinafter, referred to as “device substrate”). Separately, a light emitting layer is deposited on another substrate (hereinafter, referred to as “transfer substrate”) with a light-heat conversion layer placed therebetween. The device substrate and the transfer substrate are disposed so that the light emitting layer faces the bottom electrode, and the transfer substrate side is irradiated with a laser, so that the light emitting layer is heat-transferred onto the bottom electrode on the device substrate. In this instance, by scanning with a spot of the laser, the light emitting layer can be heat-transferred to only a predetermined region on the bottom electrode with high positioning precision (see Japanese Patent Application Publication (KOKAI) No. 2002-110350).
The use of a heat transfer method in the patterning is advantageous to scaling down of the electroluminescent element; however, the electroluminescent element obtained by this method has a problem in that the light emission efficiency and brightness-half-life are lower, as compared to the electroluminescent element manufactured by a shadow mask method.
For removing the problem, with respect to the method for manufacturing a display device using a heat transfer method, a method for improving the light emission efficiency and brightness-half-life has been proposed in which, prior to the heat transfer of the light emitting layer, the device substrate and transfer substrate (donor element) are subjected to heat treatment (see Japanese Patent Application Publication (KOKAI) No. 2003-229259). In addition, a method for improving the light emission efficiency and brightness life has been proposed in which, after the heat transfer, the display substrate is subjected to heat treatment to prevent the light emitting layer from deteriorating due to oxygen or water vapor (see Japanese Patent Application Publication (KOKAI) No. 2006-66375).